The invention relates to a pressure-fluid actuated device for applying a variable pressing force to a presser element, said device including a spring disposed in the arm head of the sewing machine.
A presser drive of this general type is disclosed in German Unexamined Application for Patent OS No. 36 04 299, in which the piston rod of a double-acting compressed-air cylinder is directly connected to the presser bar. The disclosures of this, as well as all other prior art documents cited herein, are incorporated by reference.
This device, by acting on the two cylinder spaces present in the cylinder, which action is controlled by a control unit, can change the pressing force with which the material to be sewn is pressed against a needle plate by a presser foot attached to the presser bar. However, since the piston present in the cylinder is directly connected to the presser bar, the up-and-down movement of the cloth-feed device during sewing is transmitted to the piston via the presser foot and the presser bar. The resulting highspeed movement of the piston leads to early wear of the sealing element of the piston and therefore this known presser drive is practically usable only for a few stitches at the beginning of the seam. It accordingly cannot be used for a lengthy seam.
The principal object of the invention is to provide an improved device of the foregoing type, which will permit substantially wear-free operation, even when long seams are sewed.
This object is achieved by a sewing machine having the features disclosed and claimed herein. According to a preferred embodiment, in a sewing machine having an arm head, and below the arm head at least one presser bar which raises and lowers a presser foot for applying a force to a sewing-material workpiece, the improvement of the invention comprises a device for applying a variable downward force to said presser bar, said device comprising: spring means linked to said presser bar for exerting said downward force; and servo means linked to said spring means for controllably varying a resilient force thereof and thereby varying said downward force exerted by said spring means. It further comprises compressing means actuated by said servo means for engaging said spring means and controllably varying its resilient force. The spring means may comprise a spring and said compressing means may comprise a shaft linking said servo means to said spring. Advantageously, the shaft passes through the spring, is positioned by the servo means, and engages the spring to set its length.
According to another advantageous feature, said spring is settable to a plurality of predetermined lengths, as determined by control means connected to said servo means. The control means includes piston means and comprises a valve for controlling application of a pressure medium to the piston means. The pressure medium is controllable to set said spring to a plurality of predetermined lengths.
According to a preferred embodiment, the piston means comprises at least first and second pistons and the control means controls the pistons independently of each other.
In a particularly advantageous feature, a guide lever receives the force of the spring and engages the presser bar for applying said downward force thereto.
With this device it is now possible in a very simple manner to apply different pressing forces to the presser foot during the sewing of a particular seam. In this way, in particular, a partial working in of additional width--for example when the upper or lower layer of material to be sewn is gathered--is made possible without the intermediate plate which has heretofore been indispensable for such sewing work. Up to now such an intermediate plate--as known for example from German Utility Model 84 33 111--had to be swung into the region where the stitch is formed, when performing gathering work, in order to spatially separate the two layers of material to be sewn.
These and other objects, features and advantages of the invention will be understood from the following detailed description of an embodiment thereof, with reference to the drawings.